1. Field of the Invention
This invention relates to a method and device for detecting an interface, which is installed in a vessel, such as a storage tank storing two kinds of fluids, a reaction vessel or others, or a pipe or others connected to the vessel for drawing the fluids.
2. Related Background Art
FIG. 1 shows one example of the conventional interface detecting device. A light source unit 604 and a light detecting unit 606 are opposed to each other across a pipe 602 for drawing liquids connected to the bottom of a vessel 600. The parts of the side of the pipe corresponding to the light source unit 604 and the light detecting unit 606 are made of transparent members so that the light detecting unit 606 detects a beam from the light source unit 604. A constant amount of light is emitted from the light source unit 604. When an interface 608 between an upper liquid and a lower liquid passes between the light source unit 604 and the light detecting unit 606, an output of the light detecting unit 606 deflects, and, based on this deflection, the passage of the interface can be detected.
But in the above-described interface detecting device, some kinds of liquids do not transmit sufficient light amounts to be detected by the light detecting unit when they pass through the vessel or the pipe, and in some cases an interface between an upper and a lower liquids cannot be detected. Especially in the case that both upper and lower liquids have low light transmissions when they pass through the vessel or the pipe, their interface cannot be sufficiently sensed, and the passage of their interface cannot be sufficiently detected.
In some cases, a difference in light transmission between the upper and the lower liquids is so small that a deflection or change in a detected light amount by the light detecting unit is small, with the result that the passage of the interface cannot be fully detected. That is, liquids do not have always the same light transmission. Every moment their light transmissions slightly vary. Their detected light amounts accordingly vary, and contain noise. Thus, to accurately detect an interface between an upper and a lower liquid, a sufficient difference in light transmission between the upper and the lower liquids, i.e., between detected light amounts through the respective liquids, is required. Some kinds of liquids do not make sufficient differences therebetween in the detected light amount.